Echo ultrasound is a known technology for determining the location of and for generating images from discontinuities in the value of the acoustic impedance within objects. In a typical diagnostic imaging system, a periodic train of pulses of ultrasound energy is directed into the body. Energy from the pulses is reflected from impedance discontinuities, which may occur at organ boundaries or at lesions within otherwise healthy, homogeneous tissue.
The amplitude of echoes which have reflected from the body may be plotted as a function of time to produce a A-line which represents the magnitude of impedance discontinuities at various distances along the ultrasound propagation path. If the propagation path is scanned across the body, the resultant A-lines may be combined on a two-dimensional display to produce an image of the interior of the body.
Ultrasound energy is attenuated as it passes through tissue. The magnitude of the attenuation in a local region of the body may be used to characterize the type and condition of tissue in that region and may be used, for example, to differentiate between various organs or between healthy and diseased tissue.
The ultrasonic attenuation of animal tissue is known to vary as a function of frequency and, as a result, the center frequency of a wideband ultrasound pulse will appear to shift as a function of both the tissue type and the length of the propagation path through the tissue. The apparent shift in the center frequency of ultrasound pulses reflected from a region of tissue may be used to estimate the attenuation of that tissue in a manner which is described in Ultrasonic Attenuation Tomography of Soft Tissues, Dines and Kak, Ultrasonic Imaging Vol. 1, No. 1, pages 16-33, 1979.
Apparatus which estimates the attenuation of tissue by counting zero crossings of the signal in an A-line is described, for example, in U.S. Pat. No. 4,441,368 to Flax. The apparatus described in the Flax patent is, however, sensitive to drop-outs or reductions in the amplitude of the A-line signal which may, for example, occur when ultrasound propagates through a cyst and the resultant measurements of attenuation may be inaccurate or confusing when made in the vicinity of cysts.